Passive planar frequency selective surface assemblies are known in the form of an array of identical electrically conductive elements printed on a flat dielectric substrate. Such known assemblies have the useful property, by virtue of resonances within and between the conductive elements, that the transmission and reflection of microwave energy by the surface of the assembly is strongly frequency dependent. This allows the assembly to be used in the construction of, for example multi-band reflector antennae, in which the routing and possible transmission of microwave signals is strongly frequency dependent. Thus the assembly can be used to reflect signals in one frequency band and to pass signals in another frequency band. Additional to such two layer assemblies, assemblies are also known with a plurality of layers of electrically conductive elements.
Recent research has proposed that the characteristics of a frequency selective surface assembly can be electronically altered on a very short time scale by switching or otherwise controlling active electronic devices embedded into the individual electrically conductive elements. Such an active frequency selective surface assembly could have many potential applications, but embedding electronic devices in individual electrically conductive elements is difficult and would make such assemblies expensive and difficult to make and use.